1. Field of the Invention
The present invention relates generally to a switching regulator, and is directed more particularly to a switching regulator in which the switching speed of the switching element thereof can be made high.
2. Description of the Prior Art
In the past, in order to improve the switching speed of a switching element, when the switching element is made OFF, the control potential therefor is drawn to the reverse bias direction and to rapidly discharge an electric charge or carrier, which is stored in the switching element when it is in the ON-state.
An example of the prior art switching power supply circuit or switching regulator so as to improve the switching speed is shown in FIG. 1. In the figure, a power supply plug 1, which is supplied with, for example, commercial power source voltage, is connected through a power supply switch 2 to a rectifier circuit 3 whose output side is grounded through a primary winding 4a of a transformer 4, which has a magnetic core, and a switching element, for example, collector-emitter path of a switching transistor 5. A secondary winding 4b of the transformer 4 is connected to a rectifier circuit 6 which produces a stable DC voltage for a load. The base of the switching transistor 5 is connected to the output side of a drive circuit 7 which is supplied with an electric power from a DC power supply source 8 which is, in turn, connected to the output side of the rectifier circuit 3. The drive circuit 7 includes a transistor 9, which is driven by a PWM (pulse width modulated) signal (i.e., ON-pulse) for carrying out the switching operation, a transistor 10, which is made ON in response to the output from the transistor 9, a transistor 11, which will become ON to drive the switching transistor 5 when the transistor 10 becomes ON, a transistor 13, whose base is connected to the emitter of the transistor 9 through, for example, a differentiating circuit 12 which will produce a pulse, i.e., OFF-pulse to draw the base potential of the switching transistor 5 in the reverse bias direction when it is in OFF-state, and a transistor 14, which will become ON in response to the output from the transistor 13 to connect the base of the switching transistor 5 to the negative voltage side of the DC power supply source 8.
The output side of the rectifier circuit 6 is connected to a voltage detector or detecting circuit 15 which will detect a part of the DC output voltage of the rectifier circuit 6. The output side of the voltage detector circuit 15 is connected through an isolator 16 such as a photo-coupler, transformer or the like, which will isolate the primary side of the transformer 4 from the secondary side thereof, to a pulse width modulator 17 which will compare the DC voltage delivered from the voltage detector circuit 15 with a reference voltage and control the pulse width of its output pulse by the compared error signal and hence produce a PWM signal. This PWM signal is applied to the input side of the drive circuit 7, i.e., the base of the transistor 9 to operate the drive circuit 7. The ON-time of the switching transistor 5 is controlled by the drive circuit 7, so that the rectifier circuit 6 produces at the output side thereof a stable DC output voltage.
In the prior art switching regulator or power supply circuit with the construction described as above, in order to make high the switching speed of the switching transistor 5, namely to make short the switching-OFF time of the switching transistor 5, an OFF-pulse, which draws the base potential of the switching transistor 5 in the reverse bias direction, is provided in such a manner that the ON-pulse, which is used to make the switcing transistor 5 ON, is shaped by, for example, the differentiating circuit 12 shown in FIG. 1 to provide the OFF-pulse. Accordingly, when the pulse width of the PWM signal applied from the pulse width modulator 17 to the base of the transistor 9 becomes narrow, there occurs the difficulty that the charge stored on the capacitor of the differentiating circuit 12 becomes insufficient for turning the transistor 13 ON. In other words, since the pulse width of the OFF-pulse is not sufficient, the stored carrier in the switching transistor 5 is not discharged sufficiently and hence the switching frequency can not be made high with the result that the efficiency is deteriorated.